1. Field of the Invention
This invention relates to a highly dense sintered body of silicon nitride having an improved mechanical strength and a method of producing the same, and more particularly to a highly dense silicon nitride sintered body containing predetermined amounts of Y.sub.2 O.sub.3, MgO and CeO.sub.2 and having a crystalline intergranular phase and exhibiting excellent static fatigue properties.
2. Description of the Prior Art
Silicon nitride sintered bodies are superior to metallic materials in mechanical strength at high temperature, heat resistance, thermal shock resistance, corrosion resistance and the like, so that they are considered to be used for high temperature structural components, which cannot be adapted to use metallic materials, and the development of their applications is performed extensively.
Since silicon nitride can not easily be subjected to solid-phase sintering owing to the covalent bonding substance, it is subjected to liquid-phase sintering wherein additives such as Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, MgO, CeO.sub.2, SiO.sub.2, ZrO.sub.2, rare earth oxides, AlN and the like are added to silicon nitride and a glassy phase is formed at a sintering temperature to effect densification. Thus, the resulting sintered body contains a large amount of glassy phase in its grain boundary produced during sintering thereof. Therefore, when such bodies are used in high temperature environments, the glassy phase in the grain boundary is softened to degrade static fatigue properties and oxidation resistance resulting from mechanical strength, creep deformation and creep rupture.
Consequently, many studies have been made with respect to a method for crystallizing the intergranular phase without the formation of a glassy phase. In Japanese Patent laid open No. 55-3,397, U.S. Pat. No. 4,280,850, there is disclosed a method, wherein silicon nitride is added with Y.sub.2 O.sub.3 and SiO.sub.2 and then fired to obtain a silicon nitride sintered body containing crystalline phases of Y.sub.2 O.sub.3.2SiO.sub.2 and 10Y.sub.2 O.sub.3.9SiO.sub.2.Si.sub.3 N.sub.4 in its grain boundary. In Japanese Patent laid open No. 56-59,674, there is disclosed a method, wherein silicon nitride is added with Y.sub.2 O.sub.3 and then fired to obtain a silicon nitride sintered body containing a crystalline phase of xY.sub.2 O.sub.3.ySi.sub.3 N.sub.4 in its grain boundary. Further, in Japanese Patent laid open No. 59-8,670, a silicon nitride sintered body having an intergranular phase of melilite mineral facies represented by (Si, Mg, Y)(O, N) is also shown. Furthermore, in Japanese Patent Application Publication No. 58 -50,994, there is disclosed a method, wherein a silicon nitride sintered body containing Y.sub.2 O.sub.3 or CeO.sub.3 is reheated to form a crystal of Y.sub.2 O.sub.3.Si.sub.3 N.sub.4 or CeO.sub.3.Si.sub.3 N.sub.4 in the intergranular phase. These silicon nitride sintered bodies having crystallized intergranular phases all exhibit an improved high temperature strength.
However, in order to decrease the glassy phase in the intergranular phase causing the reduction of high temperature strength, it is required to restrict the composition of additives to the composition of the crystalline intergranular phase. In this case, a sufficient amount of sintering aid can not be added to conduct densification of the silicon nitride, so that a dense sintered body is not obtained. Further, the dense silicon nitride sintered body may be obtained by hot press sintering even with the restricted composition of additives, but the hot press sintering has such drawbacks that the productivity is poor and components of complicated shape can not be manufactured. Moreover, the crystallization of the grain boundary is performed in an inert gas atmosphere during cooling from the sintering temperatures or reheating, so that the crystalline phase formed in the grain boundary is not always stable in an oxidizing atmosphere as a usual service condition. Consequently, when the sintered body is used in an oxidizing atmosphere, the mechanical strength or oxidation resistance is appreciably deteriorated due to cracks produced by locally changing the volume of the crystalline phase in the grain boundary. The silicon nitride sintered bodies, wherein the greater part of the grain boundary is composed of crystalline phase, show neither creep deformation nor creep rupture under a static load at elevated temperatures owing to no softening of glassy intergranular phase, but produce static fatigue without any deformation due to subcritical crack growth accompanied with gradual growth of initial internal defects and eventually rupture after a lapse of a certain amount of time even under a stress smaller than that corresponding to an instant fracture strength such as flexual strength or the like. Such static fatigue properties can be estimated by stress dependence of the time until the rupture of the body occurs. The subcritical crack growth resulting in the static fatigue is conceived to be affected by thermal and chemical stabilities and mechanical strength of intergranular phase; microstructural stress generated by a difference in thermal expansion between the intergranular phase and Si.sub.3 N.sub.4 crystal grains during cooling from the sintering temperature; adhesion between the intergranular phase and Si.sub.3 N.sub.4 crystal grains; properties and amount of the remaining glassy phase and so forth. Therefore, the silicon nitride sintered bodies having the crystallized intergranular phase reduce the reliability as a component material to be used under a stress for a long time due to the static fatigue rupture caused by the subcritical crack growth.